1. Field of the Invention
The present invention relates to a method and an apparatus for fabricating a crystal fiber, and more particularly, to a method and an apparatus where two external electric fields are applied on the grown crystal fiber during the growth procedure of the crystal fiber so that the growth condition can be controlled precisely.
2. Description of the Related Art
FIG. 1 shows a schematic diagram of a conventional apparatus for fabricating a crystal fiber. The conventional apparatus 10 is similar to a laser heated pedestal growth (LHPG) apparatus, which is used for making a source crystal rod 20 into a crystal fiber 21 having different regions of polarization inversion. The material of the source crystal rod 20 is lithium niobate (LiNbO3). A molten zone 16 is formed between the tip of the source crystal rod 20 and the crystal fiber 21. The conventional apparatus 10 comprises a laser beam generator (not shown), a beam splitter 12, a bending mirror 13, a paraboloidal mirror 14 and a pair of metal electrodes 18,19.
The laser beam generator is used for generating a laser beam 11. The beam splitter 12 includes an outer cone 121 and an inner cone 122. The outer cone 121 has a first conical surface 1211 and the inner cone 122 has a second conical surface 1221, respectively. The beam splitter 12 is used for splitting the laser beam 11 into a generally annular beam 111. The bending mirror 13 is used for reflecting the annular beam 111 from the beam splitter 12 and projecting it to the paraboloidal mirror 14. The paraboloidal mirror 14 is used for reflecting the annular beam 111 from the bending mirror 13, and focusing the annular beam 111 on the molten zone 16 at the tip of the source crystal rod 20. The metal electrodes 18,19 are disposed near the crystal fiber 21 and are parallel to the growth direction of the crystal fiber 21 for providing an external electric field on the molten zone 16. The metal electrodes 18,19 are connected to two high-voltage sources (not shown) respectively for providing a periodic alternating electric field so as to induce micro-swing during the growth procedure of the crystal fiber 21.
FIGS. 2a to 2c show the micro-swing occurred during the growth of the crystal fiber 21, wherein FIG. 2b shows the appearance of the crystal fiber 21 without being applied by any external electric field, FIG. 2a shows that the crystal fiber 21 swings to the left when being applied by an external electric field, and FIG. 2c shows that the crystal fiber 21 swings to the right when being applied by an external electric field. During the growth procedure of the crystal fiber 21, when the lithium niobate crystal is heated to the melting state, negative charges will be induced and distributed on the circumferences of upper portion and lower portion of the molten zone 16 because of the ionization and precipitation of the lithium ions (Li+). The negative charges are attracted by positive electric field and distracted by negative electric field, which causes the micro-swing during the growth procedure of the lithium niobate crystal fiber 21. For one crystal, its displacement is defined as the amplitude of the micro-swing of the crystal fiber 21.
FIG. 3 shows a relationship between the intensity of the external electric field and the total length of the crystal fiber 21 in the conventional apparatus 10. FIG. 4 shows a relationship between the amplitude of the micro-swing and the total length of the crystal fiber 21 in the conventional apparatus 10. The total length L1 of the crystal fiber 21 is the length of the crystal fiber 21 from the molten zone 16. If the intensity of the external electric field is constant, the value of the amplitude of the micro-swing is in direct proportion with the total length L1 of the crystal fiber 21. Accordingly, if the crystal fiber 21 grows freely without changing the intensity of the external electric field, the amplitude of the micro-swing of the crystal fiber 21 will increase continuously until the molten zone 16 breaks. Therefore, as shown in FIG. 3, when the total length L1 of the crystal fiber 21 exceeds a particular value, the intensity of the external electric field must be reduced. Additionally, it is found that the value of the amplitude of the micro-swing of the crystal fiber 21 must be larger than the diameter of the crystal fiber 21 in order to fabricate a perfect periodic polarization inversion structure. Therefore, if an external electric field adjusted according to FIG. 3 is applied to the crystal fiber 21, the value of the amplitude of the micro-swing can be controlled efficiently to be larger than the diameter of the crystal fiber 21 and to be constant, as shown in FIG. 4.
Although the value of the amplitude of the micro-swing can be controlled efficiently, the intensity of the external electric field cannot be constant and must be adjusted to a small value when the crystal fiber 21 elongates. Hence, if the intensity of the external electric field is smaller than that of the required electric field for poling, the crystal fiber 21 will not have a periodic polarization inversion structure. Therefore, the length of the periodic polarization inversion structure formed by the conventional apparatus 10 is limited.
Consequently, there is an existing need for a novel, improved method and an apparatus for fabricating a crystal fiber to solve the above-mentioned problems.